1. Field of the Invention
The present invention relates in general to high-temperature strain gages and more particularly to a temperature compensation element that includes a strain gage encapsulated in an insulative material and used to temperature compensate an active high-temperature strain gage in a half-bridge configuration. Further, the present invention relates to a method for fabricating this temperature compensation element and a high-temperature strain gage that utilizes this element
2. Background Art
All materials deform to some extent when subjected to external loads or forces. These deformations result in relative displacements that may be normalized as percentage displacement, or strain. Strain is the deformation of a material under the action of applied forces. In more precise terms, strain is the elongation of an object in some direction per unit undistorted length in some direction.
Accurately measuring the strain of a material or object is critical in a multitude of applications. Some examples of high-temperature applications where accurate strain measurement is critical are automotive engine and exhaust system testing, aircraft engine testing and nuclear reactor testing. Moreover, accurate strain measurement permits evaluation of new materials and composites so that automobile manufacturers can select a strong enough material to protect the occupants of an automobile during a collision. In addition, new materials and composites can be evaluated for use as structural members of a commercial aircraft to permit use of a material that is light enough to enable flight but strong enough to remain intact during turbulence and hard landings.
Strain may be measure in several ways. One common way is a resistive wire strain gage. The theory behind the strain gage is that when a length of wire is mechanically stretched, a longer length of a smaller sectioned conductor results, and thus the electrical resistance changes. In other words, as the wire is mechanically elongated, the electrical resistance changes. This change in resistance may be calibrated in terms of strain.
This change in resistance, however, is normally too small for ordinary resistance-measuring devices to detect. Consequently, a circuit arrangement is needed so that the minute changes in resistance may be detected. One useful circuit arrangement is a resistance bridge arrangement, which is well known in the art, and has the added advantage that it provides a means for effectively reducing or eliminating the effect of temperature on resistance.
Ideally, a strain gage would respond only to the applied strain in the part, and be unaffected by other variable in the environment. However, this is not the case, and the electrical resistance of the strain gage also varies with temperature. Moreover, the relationship between strain and resistance change varies with temperature. Especially in high-temperature applications, these deviations cause by temperature can lead to substantial error in the measured strain.
When the temperature of the strain gage changes, a subsequent change is produced in the resistance of the gage. This temperature-induced resistance change is independent of, and unrelated to, the mechanical strain that is being measured in the substrate to which the gage is attached. This resistance change is called the apparent strain, or, because it is temperature induced, the thermal output of the gage. Typically, for convenience in correcting measurement strain data, the thermal output is expressed in strain units.
Temperature compensation, therefore, is important for resistive wire strain gages to reduce the error from thermal output. Ideally, temperature compensation of a high-temperature strain gage is achieved by subtracting the thermal output from the measured strain. Typically, this is done by wiring the strain gage in a Wheatstone bridge with an active leg and an inactive leg.
In theory, the error due to thermal output can be completely eliminated by employing a half-bridge configuration that incorporates an active gage in conjunction with an inactive compensating gage. These two gage elements are then wired to form adjacent arms of a Wheatstone bridge circuit. The active gage is mounted on the substrate to "feel" the strain of the substrate, while the inactive or "dummy" gage is merely left unattached to the substrate. "Unattached" means that the gage is positioned such that the gage does not feel the strain of the substrate when the substrate is subject to strain. Both the active and inactive gages must be nearly identical and positioned such that they will both experience identical temperature changes.
Under these ideal circumstances, the thermal outputs of the two gages should be identical. And, since identical resistance changes in adjacent arms of the Wheatstone bridge do not unbalance the circuit, the thermal outputs of the active and the inactive gages should cancel exactly. Thus, when both gages are subjected to a change in temperature, the circuit remains nulled. Conversely, when the substrate is mechanically strained only the active gage responds and registers only the measured strain.
In practice, however, this method of temperature compensation is subject to errors. The principal problems encountered by this method are those of establishing and maintaining the identical conditions needed. For example, there is difficulty in ensuring that the temperature of the active and the inactive gages are always identical. Moreover, strain gages, even if from the same lot or package, are never precisely identical. Even though the differences between the gages may be negligible at room temperature, the differences may become evident and significant when used in high-temperature applications.
Several variations on the above method of temperature compensating high-temperature strain gages currently exist. One method, developed at NASA Lewis Research Center, is to use external electrical biasing of the strain gage signal in combination with a bonded inactive compensation element.
The problem, however, with this method is that it is quite expensive because costly external circuitry, including precision pots and resistors, must be used. In addition, both the signal conditioning and the data acquisition procedures are much more complex with this method. Another problem is that to optimize the strain measurements this method requires calibration. Thus, a calibration run must be made before any strain measurements can be made. This first cycle calibration severely limits the use of this method, because nearly all real world applications require first cycle strain measurements.
Another method is a remote dummy concept, whereby the inactive temperature compensation element is controlled by an oven in a remote location. The inactive gage is placed on a similar but separate piece of substrate as the active gage. The inactive gage is placed in an oven to compensate for the thermal output.
The problem with this method, however, is that the oven adds great expense to the system because expensive temperature controllers and portable mini-ovens are required. Moreover, the system is quite cumbersome to move from location to location. In addition, this method further complicates the temperature compensation procedure because thermal strains in the compensation substrate must be taken into consideration.
Another compensation method is to use a platinum element in series with a single active strain gage in order to rotate the apparent strain curve. Although this method is fairly inexpensive and simple, it is also quite inaccurate because a significant portion of the apparent strain output is not compensated. Consequently the overall thermal output compensation provided by this method is very ineffective.
A recent method developed at NASA Langley Research Center involves housing an active flame-sprayed gage and a bare inactive temperature compensation element within a ceramic enclosure. The problems with this method, however, are that it requires special gages and that its usable temperature range is quite narrow. In addition, because a ceramic housing must be built around the active and the inactive gages, the method involves a tedious and an extensive installation process. Furthermore, care must be taken when spraying down the active gage to keep the inactive gage covered.
Another problem with the Langley method concerns the apparent strain (thermal output) curve. An apparent strain curve is a graph of the thermal output of the gage versus the temperature. The slope of the apparent strain curve will shift with temperature because the curve is a function of temperature. For most materials, the slope of the apparent strain curve will become more positive as the temperature increases. Moreover, this change in slope as a function of temperature is generally non-linear. Therefore, it is usually quite difficult to compensate for the thermal output when the temperature is changing any appreciable amount.
FIG. 1 illustrates the apparent strain curves of the Langley high-temperature strain gage discussed above. Plot 130 is the two-gage average apparent strain curve of the Langley gage after a maximum test temperature of 1200 degrees F. has been attained. Plot 140 and Plot 150 are the two-gage average after maximum test temperatures of 1500 degrees and 1700 degrees has been attained, respectively. As such, each time the strain gage attains a new maximum temperature, the strain gage traces a different apparent strain curve, such as 130, 140, or 150. It can be seen that the average slope of Plots 130, 140 and 150 are not the same and in fact the average slopes change non-linearly with temperature over the entire temperature range.
Therefore, another disadvantage of the Langley method is that the slope of the apparent strain curve is a function of temperature. This makes it difficult to subtract the thermal output from the measured strain and leads to inaccuracies in the measured strain. These inaccuracies become even more significant if the temperature changes any appreciable amount.
Therefore, what is needed is a method of temperature compensation for a high-temperature strain gage capable of providing viable data in operation at temperatures in excess of 1700 degrees Fahrenheit. In addition, the operation of the strain gage would not be limited by a compensation element or a method that required special gages. Further, the strain gage would be usable over a wide temperature range.
What is further needed is a temperature compensation method for a high-temperature strain gage that is simple and easy to use. In particular, this method would not involve complicated signal conditioning or data acquisition procedures. Furthermore, this compensation method would not require tedious or extensive installation procedures.
What is further needed is a temperature compensation element and method that is effective and accurate over a wide range of high temperatures. The method must easily decouple the temperature component of strain from the measured strain to arrive at the true strain. In order to accomplish this, the method should have an virtually constant apparent strain curve slope that is fairly linear so as to permit easy canceling out of the thermal output from the measured strain.
Whatever the merits of existing and the above-mentioned temperature compensation elements and methods for high-temperature strain gages, they do not achieve the benefits of the present invention.